An internal combustion engine of an automotive vehicle comprises a valve train mechanism for actuating inlet and outlet valves synchronously to the engine rotation. The valve train mechanism generally comprises a camshaft and a cam tappet that converts the rotary motion of the camshaft into a reciprocating motion for axially operating the inlet and outlet valves. The cam tappet comprises a rocker arm that is driven by cams carried by the camshaft. With the progress of automotive vehicle technology in the field of high-performance engines, there is an increasing demand for compact and light-weight engines with a long operating life and a maintenance-free construction.
It is common knowledge in this connection that such lever-type cam followers made without chip removal from sheet steel are generally made of a case-hardening material such as, for example, 16 Mn Cr 5. Case hardening consists of carburizing or carbonitriding followed by hardening either immediately thereafter or after intermediate cooling and re-heating to an adequate hardening temperature. Depending on the required service properties or the requirements for subsequent working, hardening is followed by tempering or by sub-zero cooling and tempering. Case hardening serves to impart a substantially higher hardness to the surface layer of work-pieces made of steel and better mechanical properties to the work-pieces. For this purpose, the surface layer is enriched prior to hardening with carbon (carburizing) or with carbon and nitrogen (carbonitriding). In contrast to carburizing, the additional enrichment with nitrogen, because it modifies the transformation behavior in the surface layer, leads to a higher hardenability and, after hardening, to a higher tempering stability.
A drawback of lever-type cam followers made in this way is that the heat treatment of the case hardening material is very time-consuming and expensive.